Evaporation takes place in numerous sectors of refrigeration and air-conditioning engineering as well as in process and power engineering. In the prior art, tubular heat exchangers are often used, in which liquids evaporate from pure substances or mixtures on the outside of the tube and, in the process, cool a medium which is flowing on the inside of the tube. Such appliances are known as flooded evaporators.
By making the heat transfer on the outside and inside of the tube more intensive, it is possible to reduce the size of the evaporators considerably. As a result, the production costs of such appliances fall. Moreover, the volume of refrigerant required is reduced, which is important in view of the fact that the chlorine-free safety refrigerants which are predominantly used nowadays may form a not insubstantial portion of the overall equipment costs. If toxic or combustible refrigerants are used, reducing the volume of these refrigerants allows the potential hazard to be lowered. The tubes with passage-like structures on the outside which are customarily used nowadays are more efficient by a factor of about three than smooth tubes of the same diameter.
Prior art:
The present invention relates to a process for producing tubes with a structured outer side, the structure serving to increase the outside surface area and the heat transfer coefficient for the evaporation of liquids on the outside of the tube. In order to increase the heat transfer coefficient during evaporation, the process of nucleate boiling is made more intensive. It is known that the formation of bubbles begins at nucleation sites. These nucleation sites are generally small gas or vapor inclusions at the surface. When the growing bubble has reached a certain size, it becomes detached from the surface. If, in the course of the bubble becoming detached, the nucleation site is flooded with liquid which is continuing to flow in, under certain circumstances the gas or vapor inclusion will be displaced by liquid. In this case, the nucleation site is inactivated. This can be avoided by suitably designing the nucleation site. To do this, it is necessary for the opening of the nucleation site to be smaller than the cavity lying below it, as for example in structures of re-entrant type.
It is known to produce such structures on the basis of integrally rolled finned tubes in which the fins have been formed out of the tube wall by rolling. Integrally rolled finned tubes are understood to mean finned tubes in which the fins have been formed out of the wall material of a smooth tube. For such finned tubes to be used in tubular heat exchangers, it is in many cases necessary for the external diameter of the tube in the finned region to be not greater than the external diameter of the unfinned end sections and skip sections of the tube.
Various processes are known with which the passages situated between adjacent fins are closed off in such a manner that connections between passages and environment remain in the form of pores or slots. Liquid and vapor can be conveyed through these pores or slots. In particular, such essentially closed passages are produced by bending over or flanging the fins (U.S. Pat. No. 3,696,861, U.S. Pat. No. 5,054,548), by splitting and compressing the fins (DE-C 2,758,526, U.S. Pat. No. 4,577,381), by notching and completely compressing the fins (U.S. Pat. No. 4,660,630, EP-B 0,713,072) or by notching and compression which is offset to one side of the fins (U.S. Pat. No. 4,216,826 and the parallel DE-28,08,080 A1).
To further increase the heat transfer capacity, it is necessary to increase the outer surface area of the tube and number of passages by increasing the number of fins per unit length of the tube. In order to combine a short distance between fins with a structure of high porosity (=relative volumetric capacity proportion of the passages), it is necessary to reduce the thickness of the fins. In so doing, the processes mentioned above come up against the limits of manufacturing stability:
As the distances between adjacent fins become smaller, the tools used for flanging or bending over the fin (U.S. Pat. No. 3,696,861, U.S. Pat. No. 5,054,548) have to be made in an increasingly filigree form. Owing to unavoidable fluctuations, which lie within engineering tolerance limits, in the dimensions of the smooth tube (e.g. in the wall thickness), changes in the forces which are active during the finning process occur along the tube, resulting, when the fin is formed asymmetrically (by being bent over or flanging), in undesirable irregularities in the slot width or in the pattern of pores. As the structure becomes finer, these irregularities become increasingly serious.
In the case of thin fins, central splitting of the fin, as proposed in DE-C 2,758,526 and U.S. Pat. No. 4,577,381, can no longer be economically realized under manufacturing conditions.
Experience has shown that thin fins become bent or collapse during the compression operation if the operation is carried out as described in U.S. Pat. No. 4,660,630 and EP-B 0,713,072. It is therefore impossible to produce a structure of high porosity.
In the case of offset compression in accordance with U.S. Pat. No. 4,216,826, thin fins have a tendency to buckle to one side. Consequently, this process can only be controlled with extreme difficulty in the case of thin fins and is therefore unsuitable for mass production.
As the outer structure becomes finer, i.e. the fins become thinner, the reduction in the stability of the fin increasingly becomes the major difficulty. If the entire upper region of the fin is simultaneously deformed under the compressive load imparted by the tool, the fin collapses instead of forming a cover above the passage. It is better to break the deformation down into partial steps. DE 28,08,080 A1 already refers to this fact. In the above-mentioned document, it is proposed that the entire fin should not be deformed in a single operation, but rather the tool used for the deformation is to be arranged in such a way that only one side of the fin is deformed in one operation (cf. FIG. 2 and 14 of DE 28,08,080 A1). However, in this process the fin is deformed in such a manner that, in the case of thick fins, the upper regions of the fin become thicker, as illustrated in FIG. 17 of DE 28,08,080 A1. The thickening of the upper fin regions is explicitly mentioned in patent claim 1 of the parallel U.S. Pat. No. 4,216,826. Therefore, thin covers are not formed above the passage and the desired high level of porosity cannot be realized. In the case of thin fins, these fins tend to buckle to one side in the event of axially single-sided compressive deformation of the fin tip. For this reason, this process can only be controlled with extreme difficulty in the case of thin fins and is therefore unsuitable for mass production.
Furthermore, it is proposed, in DE 28,08,080 A1 to deform the fins using a single, suitable tool in the fashion of a gearwheel, so that after further working steps grooves are formed in the axial direction of the tube. The material which is displaced in the gearwheel-like deformation lies below the outer surface of the tube and is therefore not used to form covers above the passages between the fins. Rather, it reduces the porosity and, furthermore, prevents liquid from being conveyed in the circumferential direction in the passages.